SendMessage fra c#

Jeg vil gerne sende en message til en textbox men kan ikke rigtig få det til at virke. Her er koden:

namespace WindowsApplication7
{
   
    [StructLayout(LayoutKind.Sequential)]
    public struct RECT
    {
        public int left;
        public int top;
        public int right;
        public int bottom;
    }
    /// <summary>
    /// Summary description for Form1.
    /// </summary>
    public class Form1 : System.Windows.Forms.Form
    {
        [DllImport("USER32.DLL", EntryPoint= "SendMessage")]
        public static extern int SendMessage(IntPtr hwnd,int msg,int wParam,RECT lParam);

        const int EM_GETRECT = 0x00b2;

        private System.Windows.Forms.Button button1;
        private System.Windows.Forms.TextBox textBox1;
        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.Container components = null;

        public Form1()
        {
            //
            // Required for Windows Form Designer support
            //
            InitializeComponent();

            //
            // TODO: Add any constructor code after InitializeComponent call
            //
        }

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        protected override void Dispose( bool disposing )
        {
            if( disposing )
            {
                if (components != null)
                {
                    components.Dispose();
                }
            }
            base.Dispose( disposing );
        }

        #region Windows Form Designer generated code
        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.button1 = new System.Windows.Forms.Button();
            this.textBox1 = new System.Windows.Forms.TextBox();
            this.SuspendLayout();
            //
            // button1
            //
            this.button1.Location = new System.Drawing.Point(64, 16);
            this.button1.Name = "button1";
            this.button1.TabIndex = 0;
            this.button1.Text = "button1";
            this.button1.Click += new System.EventHandler(this.button1_Click);
            //
            // textBox1
            //
            this.textBox1.Location = new System.Drawing.Point(40, 88);
            this.textBox1.Multiline = true;
            this.textBox1.Name = "textBox1";
            this.textBox1.Size = new System.Drawing.Size(184, 48);
            this.textBox1.TabIndex = 1;
            this.textBox1.Text = "textBox1";
            //
            // Form1
            //
            this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
            this.ClientSize = new System.Drawing.Size(292, 273);
            this.Controls.Add(this.textBox1);
            this.Controls.Add(this.button1);
            this.Name = "Form1";
            this.Text = "Form1";
            this.Load += new System.EventHandler(this.Form1_Load);
            this.ResumeLayout(false);

        }
        #endregion

        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.Run(new Form1());
        }

        private void Form1_Load(object sender, System.EventArgs e)
        {
       
        }

        private void button1_Click(object sender, System.EventArgs e)
        {
            RECT rc = new RECT();

            try
            {
                SendMessage(textBox1.Handle,EM_GETRECT,0, rc);
            }
            catch(Exception ex)
            {
                string h = "";
            }
        }   
    }